Well servicing fluids, such as completion fluids or packer fluids, are often salt brines with various concentrations of organic solvents. These fluids can be designed to have a specific density so as to balance the weight of the completion or packer fluid column with the well formation pressure. If the fluid is too heavy, the fluid can sink into the well, possibly plug up the formation and may be difficult to recover. In order to form well servicing fluids having a fluid density that will be balanced with respect to the formation pressure, it is often desirable to employ low density fluids as a component of, for example, the well completion fluid.
In addition to being balanced, it is desirable that well servicing fluids also be protected against gas hydrates. The formation of hydrates in well servicing fluids is a well known problem. Gas hydrates are crystalline compounds that occur when water forms a cage-like structure around guest molecules, particularly gaseous molecules. In the petroleum industry, gas hydrates pose particular problems with respect to producing, transporting, and processing of natural gas in petroleum fluids. Typical gas hydrates formed in petroleum producing and transporting environments are composed of water and one or more guest molecules such as methane, ethane, propane, isobutane, normal butane, isopentane, normal pentane, nitrogen, carbon dioxide, and hydrogen sulfite. It is also known that other guest molecules such as ethylene, nitrous oxide, acetylene, vinyl chloride, ethyl bromide, oxygen, etc., can also form hydrates.
A number of solutions are known for reducing gas hydrates. For example, high concentration salt solutions can be very efficient thermodynamic hydrate inhibitors. While these salts can be useful for preventing the formation of hydrates under some conditions, the increased brine concentrations also generally increase the density of the fluid. This can cause the fluid to become undesirably overbalanced, making it unsuitable for some well applications.
In ultra-deep offshore waters, low density completion and/or packer fluids are often used. The low density fluids are generally designed to avoid damaging oil and gas bearing formations and to not impede future gas or oil output from the well. In a typical deepwater oil/gas well, such fluids must function under significant pressures and low mudline temperatures. Such conditions create a favorable environment for the formation of gas hydrates plugs in the low density completion or packer fluids.
One known solution for providing a low density fluid that is resistant to hydrate formation is to use various low density alcohols, which can shift hydrate formation toward lower temperature and higher pressure conditions. For example, methanol, ethanol, glycols, glycol ethers and polyglycols are well known thermodynamic hydrate inhibitors. These additives thermodynamically destabilize hydrates and effectively lower the temperature of hydrate formation. Conventionally, a large quantity of alcoholic solvent is added into a low density brine to achieve the desired gas hydrate inhibition properties under extreme conditions, such as deepwater oil/gas well completions. The addition of such alcoholic solvents inevitably increases complexity of fluid placement and causes greater safety and environmental concerns. This is at least in part due to the highly flammable and potentially explosive nature of alcohols, such as methanol and ethanol. Significant cost increase is also often associated with the use of alcoholic solvents in such situations.
Another known solution for reducing problems caused by hydrates is to use crystal growth inhibitors to inhibit the formation of the gas-hydrate crystals and/or the agglomeration of the gas hydrate crystallites to avoid forming crystalline masses sufficiently large to cause plugging. Examples of well known crystal growth inhibitors include surface active agents such as phosphonates, phosphate esters, phosphonic acids, salts and esters of phosphonic acids, inorganic polyphosphates, salts and esters of inorganic polyphosphates, polyacrylamides, and polyacrylates.
It is also known that poly-N-vinyl-2-pyrrolidone (PVP), which is a well-known water-soluble polymer, is effective, in relatively low concentrations, in interfering with the growth of gas-hydrate crystals. WO94/12761, published Jun. 9, 1994, discloses other additives for inhibiting crystal growth and controlling the formation of gas hydrates in fluid mixtures such as are encountered in the hydrocarbon industry in the production, transportation, and processing of petroleum and natural gas fluids. The crystal growth inhibitors are often called kinetic hydrate inhibitors (KHI). They are applied at low concentrations and prevent hydrate formation for a limited time, normally not beyond a few days. Thus, KHI are generally not used if the fluid has to be protected from hydrates for a longer period of time.
Completion fluids have also been protected from hydrate formation with combinations of inorganic salts and organic solvents. However, metal salt and glycol solutions that are effective against hydrates are heavy and often result in an overbalanced fluid that is too heavy and would be lost in the hydrocarbon producing formation.
The fluids of the present disclosure may provide improvements related to one or more of the problems with conventional well servicing fluids.